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Hindlimb Function in the Alligator: Integrating Movements, Motor Patterns, Ground Reaction Forces and Bone Strain of Terrestrial Locomotion Stephen M

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Hindlimb Function in the Alligator: Integrating Movements, Motor Patterns, Ground Reaction Forces and Bone Strain of Terrestrial Locomotion Stephen M Clemson University TigerPrints Publications Biological Sciences 2004 Hindlimb function in the alligator: integrating movements, motor patterns, ground reaction forces and bone strain of terrestrial locomotion Stephen M. Reilly Jeffrey S. Wiley Audrone R. Biknevicius Richard W. Blob Clemson University, [email protected] Follow this and additional works at: https://tigerprints.clemson.edu/bio_pubs Recommended Citation Please use publisher's recommended citation. This Article is brought to you for free and open access by the Biological Sciences at TigerPrints. It has been accepted for inclusion in Publications by an authorized administrator of TigerPrints. For more information, please contact [email protected]. The Journal of Experimental Biology 208, 993-1009 993 Published by The Company of Biologists 2005 doi:10.1242/jeb.01473 Hindlimb function in the alligator: integrating movements, motor patterns, ground reaction forces and bone strain of terrestrial locomotion Stephen M. Reilly1,*, Jeffrey S. Willey1, Audrone R. Biknevicius3 and Richard W. Blob2 1Department of Biological Sciences, Ohio University, Athens, OH 45701, USA, 2Department of Biological Sciences, Clemson University, Clemson, SC 29634, USA and 3Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, OH 45701, USA *Author for correspondence (e-mail: [email protected]) Accepted 21 December 2004 Summary Alligator hindlimbs show high torsional loads during during stance phase in sprawling and erect quadrupeds terrestrial locomotion, in sharp contrast to the bending or act in isometric or even eccentric contraction in alligators, axial compressive loads that predominate in animals that stabilizing the knee and ankle during the support-and- use parasagittal limb movements. The present study propulsion phase. Motor patterns in alligators reveal the integrates new data on hindlimb muscle function with presence of local and temporal segregation of muscle previously obtained data on hindlimb kinematics, motor functions during locomotion with muscles that lie side by patterns, ground reaction forces and bone strain in order side dedicated to performing different functions and only to (1) assess mechanisms underlying limb bone torsion one of 16 muscles showing clear bursts of activity during during non-parasagittal locomotion in alligators and (2) both stance and swing phases. Data from alligators add to improve understanding of hindlimb dynamics during other recent discoveries that homologous muscles across terrestrial locomotion. Three dynamic stance phase quadrupeds often do not move joints the same way as is periods were recognized: limb-loading, support-and- commonly assumed. Although alligators are commonly propulsion, and limb-unloading phases. Shear stresses due considered models for early semi-erect tetrapod to torsion were maximized during the limb-loading phase, locomotion, many aspects of hindlimb kinematics, muscle during which the ground reaction force (GRF) and activity patterns, and femoral loading patterns in caudofemoralis (CFL) muscles generated opposing alligators appear to be derived in alligators rather than moments about the femur. Hindlimb retraction during reflecting an ancestral semi-erect condition. the subsequent stance-and-propulsion phase involves substantial medial rotation of the femur, powered largely by coordinated action of the GRF and CFL. Several Key words: locomotion, kinematics, kinetics, bone strain, motor muscles that actively shorten to flex and extend limb joints patterns, alligator. Introduction Terrestrial tetrapods use diverse limb postures during broad patterns in the mechanics and dynamics of tetrapod limb locomotion, ranging from the highly sprawling posture of movements (e.g. Cavagna et al., 1977; Biewener 1989, 1990; salamanders and some lizards (in which the limbs are held out Alexander and Jayes, 1983; Hildebrand, 1976; Demes et al., to the side of the body; Ashley-Ross, 1994a,b; Reilly and 1994; Blickhan and Full, 1992). However, an increasing DeLancey; 1997a,b; Rewcastle, 1981) to the parasagittal number of studies on limb function in tetrapods using non- posture of birds and many mammals (in which the limbs are parasagittal limb postures have identified major differences in held more directly beneath the body; Jenkins, 1971; Reilly, limb mechanics between animals that use parasagittal limb 2000; Gatesy, 1990). Between these extremes, several species movements versus those that use non-parasagittal limb use intermediate postures (Jenkins, 1971; Gatesy, 1997; movements (Blob and Biewener, 1999, 2001; Reilly and Blob, Pridmore, 1985), and some, such as crocodilians and some 2003; Parchman et al., 2003; Willey et al., 2004). Among the lizards (e.g. iguanas), are capable of using a range of limb foremost of these differences is the contrasting loading regimes postures even over a restricted range of speeds (Gatesy, 1991; found in the limb bones. Although there are some exceptions Reilly and Elias, 1998; Blob and Biewener, 1999, 2001; Reilly (e.g. the femora of chickens; Carrano, 1998), the limb bones and Blob, 2003). Studies of several different aspects of of species using parasagittal limb movements are primarily terrestrial locomotion in mammalian and avian species using loaded in bending or axial compression (Alexander, 1974; parasagittal limb postures provided a foundation for discerning Rubin and Lanyon, 1982; Biewener et al., 1983, 1988). In THE JOURNAL OF EXPERIMENTAL BIOLOGY 994 S. M. Reilly and others contrast, torsional loads exceed bending loads in species using bipedal species (kangaroos, birds, humans). To date, no study non-parasagittal posture that have been examined to date has combined analyses of locomotor forces, limb bone loads, (alligators and iguanas; Blob and Biewener, 1999, 2001). motor patterns, kinematics and foot fall patterns to evaluate Though torsional loads are clearly present in the limb bones how a non-erect quadrupedal animal supports its body weight of the non-parasagittal species that have been tested, the and generates propulsive forces with its limbs. underlying causes of this torsion remain to be clarified. In the In this study, we present an integrative analysis of few vertebrates that have been found to exhibit strong torsional propulsive dynamics in the hindlimb of the American alligator loading of limb bones (e.g. walking chickens, flying bats and (Alligator mississippiensis Daudin). By integrating data on birds; Pennycuick, 1967; Swartz et al., 1992; Biewener and limb forces, bone loads, kinematics and motor patterns, we are Dial, 1995; Carrano, 1998), these torsional loads are believed able to evaluate specific questions about locomotor mechanics to be induced by locomotor forces acting at a distance from the in this species. In particular, we assess the potential long axis of the limb bone which, therefore, generate a mechanisms underlying limb bone torsion during non- torsional moment. In species using non-parasagittal terrestrial parasagittal locomotion in alligators. Additionally, our locomotion, two different forces are likely to act at a distance analyses allow us to refine evaluations of limb muscle function from limb bone long axes and potentially contribute to and the roles that these muscles play in supporting body weight torsional moments. First, because species using non- and retracting the limb. Based on limb movement, force and parasagittal locomotion hold their limbs out to the side of the motor pattern data, we distinguish three dynamic phases during body for much of limb support, the ground reaction force hindlimb stance: a limb-loading phase, a support-and- (GRF) will be directed either anterior or posterior to the long propulsive phase, and a limb-unloading phase. Comparisons of axis of limb bones for most of the step in these animals (Blob locomotor patterns among tetrapods suggest that limb function, and Biewener, 1999, 2001). At the same time, the GRF will be forces, and the use of femoral rotation in alligators may be directed more nearly perpendicular to the femur of non- quite different from patterns understood for other species that parasagittal animals than it will be in parasagittal animals. As use either more sprawling or more erect limb posture. These a result, the GRF will tend to rotate limb bones about their long results suggest that species using non-parasagittal posture may axis in these species, contributing to torsional loading. Second, exhibit considerable diversity in their locomotor mechanics. the major limb retractor muscle in many non-parasagittal Furthermore, tail dragging has been shown to have serious species (including alligators and iguanas) is the caudofemoralis consequences for locomotor mechanics in alligators (Willey et longus (CFL), which inserts on the ventral surface of the al., 2004) which are borne out in patterns of femoral function proximal femur (Snyder, 1962; Reilly, 1995; Gatesy, 1997). in this study. Thus, distinctive features of locomotor dynamics When this muscle contracts to retract the leg during the stance in alligators may be a consequence of dragging the tail rather phase of locomotion, it acts at a distance from the long axis of than general features of non-parasagittal postures. the femur equal to the radius of the bone, and this distance is a moment arm for long axis rotation of the femur. Thus, contraction of CFL should produce inward (medial) rotation of Materials and methods the femur and torsional loading. This analysis re-examines and synthesizes data that were Though both
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